After weeks of speculation, Democratic U.S. Rep. Stephen Lynch of South Boston launched his bid for a U.S. Senate seat representing Massachusetts, declaring his candidacy in an upcoming special election and kicking off an intra-party primary battle between himself and U.S. Rep. Ed Markey of Malden, Mass.

Lynch kicked off his campaign with a Web video, "Stephen Lynch for U.S. Senate." The video highlighted Lynch's humble upbringing - he grew up in public housing, the son of a postal worker and an iron worker - and talks about his strong support for Social Security and Medicare, as well as his opposition to the TARP program.

Markey has already received several big endorsements - including an endorsement from John Kerry, who is vacating the Senate seat to become U.S. secretary of state.

Markey released a statement on Thursday afternoon welcoming his challenger, and encouraging him to agree to keep third party groups out of the race, following the example of November's Senate race in which Elizabeth Warren unseated then-Sen. Scott Brown.

"I welcome Stephen Lynch into the race for U.S. Senate," the statement read. "I urge him to join me in committing to the people's pledge to prevent outside special interests groups from injecting millions into this campaign."

The two men will face off in a primary on April 30.

On the Republican side, former Sen. Brown has made no official indication on whether he'll make a run for Kerry's seat, but The Associated Press has reported that he is "leaning strongly" towards entering the race.